An unthinking or insane leftist -- in other words, most modern leftists.

Moonbat can also be used as an adjective, e.g. a moonbat professor. According to the Wikipedia entry for moonbat, the word was coined in 2002 by the Editor of Samizdata, Perry de Havilland, and was a variation on the name of radical British activist and columnist George Monbiot.

Originally, the term "moonbat" was intended to be more politically neutral, and described wackos on the left and the right, but it quickly acquired its current usage of being applied almost exclusively to those on the left.

The term also references the moon much in the same way that "lunatic" refers to the insanity-causing powers of the full moon (luna = moon). Bloggers occasionally analyze the behavior patterns of various moonbat "species" as if they were actual animals, and even give them satirical Linnaean taxonomical names, such as "moonbattus berkeleyensis".

The entry in the Samizdata glossary indicates that Perry originally coined the full phrase "barking moonbat"; apparently "moonbat" is just a subsequent shortened version of "barking moonbat," rather than being a pre-existing term that was lengthened to barking moonbat.

Worker #1 "Did you see those moonbats on the news staging another one of their useless protests?"

Someone way over on the political left that makes insane, irrational statements about their opposition on the right, Example of an actual moonbat statement: "These extremist Tea Baggers (moonbat's pornographic name for Tea Party) were the reason why S&P lowered the US credit rating". So a group of people that are trying to get our politicians to spend within its means are "extremists" and the cause of the downgrade when the S&P explicitly stated the US needs to cut even more than what the TP has advocated ? Yep, makes sense to me. Welcome to the irrational, upside-down world of the moonbat.

I can't talk to my moonbat neighbor because every other word out of his mouth is Bush sucks, Cheney blows and of course the axis of Obama/Reid/Pelosi are the best thing since sliced bread.

An extreme leftist, throwing aside logic for political
convenience. A caller to the Howie Carr radio show summed
it up as "These people wear winter clothes in the summer".
Characterized by men in ponytails, sandals with socks,
and 1.20.09 Bush's Last Day bumperstickers. Often seen
at global warming marches that take place during snowstorms.

A mentally unstable Liberal who tires to hijack the true meaning of a word and turn it to their advantage. Just as they have with History.
The true definition of Moonbat is not a compliment to Liberals. It is a term used to describe their mental instability and lack of knowledge.

If you tell the same lie over and over again eventually people will start to believe it. Moonbats are notorious for telling those lies and for believing them. Look at the actual history of Congressional voting record. The Liberals have a bad voting history. They were the biggest opposition to the abolishment of slavery. Yes the Republicans freed the slaves. Liberals were the biggest opposition to women’s rights and they were the biggest opposition of the Civil Rights Act.
Their Congressional voting history proves it.
Moonbats need to pay attention to the history of the KKK. It was the so called liberals who founded the KKK. Yes that is right, it was founded by so called Democrats and they have a long documented history in the KKK.Liberals seem to have forgotten the KKK was founded in the south and back then the south was all Democrat.
These are all examples of how they have told the same lies for so long that mentally unstable people believe their lies.

A pseudo-intellectual progressive type who would be so retarded and or misinformed or so duped by all the hype as to turn our country over to a JUNIOR senator from Illinois who has spent the better part of his term as Senator pimping himself to the American public when he should be proving himself on a senatorial level, retards would include: Hollywood, minorities, college age, first time voters, the cause oriented, Hamas, The Black Panther Party, Nation of Islam.

This country blew its last best chance by not getting behind Ron Paul, now watch as it goes down the toilet in a socialistic, entitlement oriented swirl! Moonbat!